1. Field of the Invention
The invention pertains to the field of semi-conductors and more particularly to a heterojunction field-effect transistor with a quick response time, this response time being determined no longer by a variation in the number of the carriers of the channel but by a variation in the speed of these carriers.
2. Description of the Prior Art
Generally, in field-effect transistors, the conductivity between two contacts, a source and a drain contact, is controlled by the voltage applied to a third contact known as the gate. Conventionally, in these field-effect transistors, the conductivity is controlled by the variation in the number of charge carriers under the gate, this variation being monotonous with the gate voltage.
For example, in a MESFET type transistor, the thickness of the conductive channel increases when a positive voltage is applied to the gate . Similarly, in an insulated gate MOSFET or a heterojunction TEGFET, the density of the charges under the gate increases monotonously with the voltage applied to the gate.
The European patent application No. 0091831 has described a field-effect transistor where the conductivity is determined no longer by the variation in the number of carriers under the gate but by the variation in the speed of these carriers. The solution proposed therein consists in creating a channel, in a semi-conductive layer, between two semi-conductive layers with a greater forbidden gap; by introducing a small number of donor impurities on one side of the channel and a large number of donor or acceptor impurities on the other side of the channel. The conductivity of this channel depends on the distribution of the charges. A normal gate and an additional gate placed in rear contact can be used to control the mean position of the distribution of charges in this canal. As a result, the mobility of the carriers can be modulated, without any variation in the number of these carriers, owing to the fact that these carriers are either in a high mobility zone or in a low mobility zone. This principle of operation has been clearly demostrated but the size of the effect is very low even at very low temperatures, for example, at a liquid helium temperature of 4.degree. K.